Production of chloride-free potassium phosphates

ABSTRACT

A PROCESS FOR THE PRODUCTING OF CHLORIDE-FREE POTASSIUM PHOSPHATE FERTILIZERS, CHLORINE AND HYDROGEN, INCLUDING A METHOD OF TREATMENT OF IMPURE SATURATED POTASSIUM CHLORIDE SOLUTION WHEREIN A PURE OR IMPURE BRINE IS ELECTROLYZED AT HIGH CURRENT EFFICIENCY. CHLORINE IS FORMED AT THE ANODE AND POTASSIUM AMALGAM, AT THE CATHODE. SAID AMALGAM IS SUBSEQUENTLY REACTED WITH ORTHOPHOSPHORIC ACID OR POLYPHOSPHORIC ACID SOLUTION AND WATER TO PRODUCE HYDROGEN AND POTASSIUM ORTHOPHOSPHATE OR POTASSIUM POLYPHOSPHATE SOLUTION, SOLIDS, AND SOLUTION WITH SOLIDS; OR AMALGAMS OF OTHER METALS, SUCH AS SODIUM, LITHIUM, AND ZINC ARE PRODUCED AT HIGH CURRENT EFFICIENCY FROM THEIR RESPECTIVE SIMILARLY TREATED IMPURE CHLORIDE BRINE FOR SUBSEQUENT PROCESSING.

Aug. 27, 1974 T. P. HIGNETT ETAL PRODUCTION OF CHLORIDE-FHEE POTASSIUM EHOSPHAIES Filed June 18, 1973 5 Sheets-Sheet 1 52g Q .92 O oramol Sanoma zoom ZOFDJOm m30 um mPODoOm 9.5m Al MESES@ uw mzmm OMIM-imo Aug. 21,v 1914 T p, HIGNETT UAL 3,832,154

PRODUCTION OF CHLORIDE-FREE POTASSIUM PHOSPHATES Filed June 18. 1975 5 Sheets-Sheet 2 PHOSPHORIC PRoD'ucT so LuTloN MAGNETIC STIRRER AND HEATER DECOMPOSER Fi q. 2

EXPLORATORY BENCH-SCALE BATCHv CHLOR-PHOSPHATE SYSTEM Aug. 27, 1974 T P, HlGNET-r ETAL 3,832,154

PRODUCTION OF CHLORIDEFREE POTASSIUM PHOSPHATES Filed June la, 197s 5 shaetssneex 5 KOH HCI pH ADD A 1 V SOLID GE cl STK PUMP PRODUCT y l MAGNETIC MAGNEUC STIRRER s'nRRER u 3 SEMI-CONTINUOUS CHLOR- PHOSPHATE SYSTEM Aug. 27, 1974 11 p, HlGNETT EITAL 3,832,154

PRODUCTION 0F CHLoRIDE-FREE POTASSIUM PHOSPHAIES 5 Sheets-Sheet A Filed June 18. 1973 BRINE SATURATOR PUMP PUMP

SATURATED KCI CONTROL QUALITY PHOS PHATES Pi q. 4 BENCH-SCALE CHLoR-PHOSPHATE SYSTEM Aug. 27, 1974 fr, P, vi-qlcgNE'Vr ETAL A 3,832,154

PRODUCTION OF CHLORIDE-FREE POTASSIUM FHOSPHATES Filed June 1s, 1973 KWG) 5 sheets-sheet s .m HOT f v WATER TO SORGE y PROCESS TANK HOT WATER WATER l LA LJ MAGNETIC t .g PHOSPHATES SPIN BAR I Hg MAGNETTO sTnRRER ANO HOT PLATE Fig. 5

DEOOMPOSER 3,832,154 Patented Aug. 27, 1974 3,832,154 PRODUCTION F CHLORlDE-FREE POTASSIUM PHOSPHATES Travis P. Hignett, Sheiiield, and Anthony J. Smith, Florence, Ala., assgnors to Tennessee Valley Authority system KZO-PO5-H2O of mixtures of the orthophosphates and the short-chain phosphates have not been established,

Continuation-impart of application Ser. No. 296,296, Oct.

10, 1972. This application June 18, 1973, Ser. No.

Int. Cl. C01d 11/00; C05b 7/00 U.S. Cl. 71-34 5 Claims ABSTRACT OF THE DISCLOSURE A process for the production of chloride-free potassium phosphate fertilizers, chlorine and hydrogen, including a method of treatment of impure saturated potassium chloride solution wherein a pure or impure brine is electrolyzed at high current eiiiciency. Chlorine is formed a-t the anode and potassium amalgam, at the cathode. Said amalgam is subsequently reacted with orthophosphoric acid or polyphosphoric acid solution and water to produce hydrogen and potassium orthophosphate or potassium polyphosphate solution, solids, and solution with solids; or amalgams of other metals, such as sodium, lithium, and zinc are produced at high current efficiency from their respective similarly treated impure chloride brines for subsequent processing.

The invention herein described may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty there- This application is a continuation-impart of our copending application Ser. No. 296,296, led Oct. 10, 1972, for Production of Chloride-Free Potassium Phosphates.

Our invention relates to an improved process for the production of potassium phosphates, potassium hydrogen phosphates, mixtures thereof, and solutions thereof, said phosphates including both orthophosphates and condensed phosphates and derived from the electrolysis of potassium chloride to form potassium amalgam which in turn is reacted with phosphoric acid. A second embodiment of our invention relates to a brine treatment which permits the electrolysis of impure sodium or potassium chloride solution in a mercury cell to produce amalgam at a high current eciency.

The term phosphoric aci used in the specifications and claims is defined as orthophosphoric acid or any mixture of orthophosphoric and condensed (poly) phosphoric acids; the general formula HMZPnO'anH, and the corresponding oxide formula n being the number of phosphorus atoms in the linear molecule, are applicable. The potassium phosphates of phosphoric acid have the general formula l being the number of molecules of Water of hydration (rather than water of constitution), and m being the number of atoms of potassium in the molecule.

The chloride present in potassium chloride is useless in agriculture, and for some crops and soils it is highly objectionable; on the other hand, however, potassium phosphates and potassium hydrogen phosphates are excellent high-analysis, chloride-free fertilizers. 'Ihe orthophosphates and the short-chain condensed phosphates are very soluble and deliquesce. Although the solubilities in the the solubilities of potassium orthophosphates (J. R. Van Wazer, Phosphorus and Its Compounds, Vol. 1, p. 496, Interscience Publishers, Inc., New York, N.Y., 1958) serve asa guide and show that at 50 C. liquids containing as much as 70 percent plant Vfood can be obtained. The same reference (page 496) states that mixing orthophosphates with pyrophosphates increases the solubilities of the potassium phosphates. It was also reported [Frazier,

. A. W., Scheib, R. M., and Lehr, I. R., Agricultural and Food Chemistry, 20, 146-50 (1972)] that the solubilities of mixtures of potassium orthoand pyrophosphate were greater than those of the orthophosphates alone. And it was also reported [Farr, T. D., and Williard, I. W., Journal of Chemical Engineering Data, I6, 67-8 (1971)] thatmixtures of ammonium ortho, pyro, and tripolyphosphates are more soluble than mixtures of orthoand pyrophosphates, which, in turn, are more soluble than orthophosphates alone. Thus, even at 0 C., stable or metastable liquids containing approximately 70 percent plant food as KZC and P205 can probably be obtained. Further, if desired, water can be evaporated from these liquids or from solid potassium phosphates separated therefrom to produce solid potassium phosphate fertilizers of even greater plant-food content.

Potassium phosphates may be made by reaction of potassium chloride with phosphoric acid, but the reaction is incomplete except at high temperatures. A byproduct of the reaction is hydrogen chloride or hydrochloric acid which is very corrosive and presents difficult disposal problems. Even if neutralized with limestone, the resulting calcium chloride is dilicult to dispose of without pollution of surface or ground waters. While hydrochloric acid has some industrial uses, its value is low, and the cost of transportation often exceeds its value. It may be noted that Belgium Pat. 739,119 claims an economic process whereby potassium chloride reacts with excess sulfuric acid, and the resulting slurry reacts with phosphate rock to produce a fertilizer-grade potassium phosphate. Coproducts are hydrochloric acid and phosphoric acid; byproducts are calcium sulfate and fluorine compounds.

Potassium phosphates may Ibe produced by reaction of phosphoric acid with potassium carbonate or hydroxide, but the cost of these potassium compounds is so high that these processes have only limited usefulness for fertilizer purposes. Further, a 50-percent potassium hydroxide solution, concentration commonly produced, is capable of reacting with phosphoric acid to produce fertilizer liquids of less than 55 percent plant food unless evaporation of water can be justified.l Although potassium metal would react directly with phosphoric acid to produce chlovride-free potassium phosphates, such a process would be uneconomic and difiicult to control.

Alkali metal polyphosphates have been used in electrolytic processes to limit corrosion of the anode (Ger.

Offen. 1,928,748; 1,957,162) and in an electrolytie process to clean copper alloy (Ger. Oien. 1,919,365). lSodium polyphosphate has been used in galvanic cells to impro/ve the electrical characteristics of an electrolyte containing NH4C1 and HgClz (U.S.S.R. 208,781). Phosphoric acid has Ibeen used in the alkaline electrolysis of sea water or brine in a diaphragm cell for the simultaneous production of chlorine and solid phosphatic fertilizer containing calcium, magnesium, potassium, and sodium phosphates (Japan 70/37,051; British 1,247,198).

In the amalgam process for the production of caustic and chlorine, an aqueous solution of a chloride salt is electrolyzed. Chlorine gas is evolved at the anode and the metal of the salt is deposited on a flowing mercury cathode. The metal dissolves in the mercury, forming an amalgam which is drained from the electrolysis cell. Hence the electrolysis of potassium chloride produces'potassiur'nl amalgam; potassium as amalgam is less expensive than potassium metal. `A comprehensive review of the uses that are put to such amalgams is reported by Robert B. MacMullin in Chemical Engineering Progress, 46, No. 9, 440-55 (September 1950). Probably the principal use of potassium amalgam is by Chlor-alkali plants in production of potassium hydroxide by reaction with water; graphite or other short-circuiting material is required in the decomposer to make the reaction proceed.

We have now discovered a simplified method of preparing chloride-t'ree potassium phosphates; several new and advantageous features over the processes shown in the prior art are realized by the present invention. Among these advantageous features are: a process which (l) is convenient for preparing and handling potassium phosphates; (2) is relatively simple and convenient for permitting the production of a composition or compositions of matter which are not now available on a commercial basis, but which are very suitable for use as high-analysis fertilizers and as components in mixed fertilizers; (3) utilizes commercially available raw materials; (4) produces a valuable coproduct, chlorine, and byproduct, hydrogen, of commercial importance; (5) produces chlorine without the constraint of the production of coproduct caustic; (6) obviates the requirement of the use of graphite or other short-circuiting material presently needed for the reaction of amalgam with water; and (7) permits the electrolysis of impure brine in a mercury cell, yet achieves a high current efiiciency for the production of amalgam. t

It is therefore an object of the present invention t produce economically, simply, and directly potassium phosphates as concentrated aqueous solution, or as mixture of solution and solids, or as mixture of solution and mixture of solids, or as particular phosphate, or as a mixture of particulate phosphates, all essentially free of chlorine.

Another object of the present invention is the economical, simple, and direct production of potassium phosphates by a process which utilizes commercially available, relatively inexpensive and well-known chemicals as raw materials.

Still another object of the present invention is the production of a valuable coproduct, chlorine, and of a valuable byproduct, hydrogen With the hydrogen available for sale or for such in-plant use as the production of ammonia A further object of the present invention is the elimination of the constraint imposed when potassium is amalgamated and reacted with water: coproducts are chlorine and caustic; the marketing outlook for each product within a given region seldom matches production of both products.

A still further object of the present invention is the elimination of the necessity of reacting the amalgam in a short-circuited decomposer packed with graphite.

Another object of the present invention is the preparation of potassium phosphate solutions ymore concentrated than those it is possible to prepare without evaporation of water by reacting commercial 50 prccent caustic with phosphoric acid.

Another object of the present invention is the economic electrolysis of sodium chloride or potassium chloride solutions containing greater concentrations of impurities than can normally be tolerated in a mercury cell.

In carrying out the objects of our invention in one form thereof, We electrolyze potassium chloride solution in a conventional mercury-cathode cell and produce chlorine and potassium amalgam which are removed from the cell. The amalgam flows into a decomposer where it reacts with suitable proportions of phosphoric acid and water to produce hydrogen gas and a solution of potassium phosphates or a solution with precipitated potassium phosphate solids. The precipitated solids can be separated as product, or a mixture of solution and precipitated solids can be removed as product, or a solution with no precipitated solids can be removed as product. We have further fround that the reaction rate of the amalgam with the acid and Water is so great that it is unnecessary to use graphite or other short-circuiting material in the decomposer to accelerate the reaction as is the prior art practiced in the Chlor-alkali industry. We have also found that the reaction will take place at a controllable and convenient rate in an acid, a neutralized or an over-neutralized solution (pH 10 or below), permitting production of a product of desired acidity simply by suitably proportioned admixture of reactants. In addition, we have found that when the reaction is adequately controlled the distribution of phosphate species is essentially unchanged when the reactant acid is converted to phosphate product.

In another form of carrying out the objects of our invention, we electrolyze either sodium chloride or potassium chloride solution containing a greater concentration of impurities than can normally be tolerated in a mercury cell, yet obtain high current etiiciencies for the production of sodium or potassium amalgam.

Still further and more general objects and advantages of the present invention will appear from the more detailed description set forth below, it being understood, however, that this more detailed description is given by way of illustration and explanation only and not by way of limitation, as various changes therein may be made by those skilled in the art without departing from the spirit and scope of the present invention.

Our invention, together with further objects and advantages thereof, will :be better understood from a consid- `eration of the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a liowsheet illustrating the principles utilized in carrying out our invention.

FIG. 2 is a diagram of the equipment we have used on exploratory bench-scale batch-tests of our process.

FIG. 3 is a diagram of the equipment we have used on semi-continuous tests of our process.

FIG. 4 is a diagram of the equipment we have used on larger, bench-scale continuous tests of our process.

FIG. 5 is an illustration of a reactor used in conjunction with additional bench-scale continuous tests of our process.

Referring now specifically to FIG. 1, solid potassium chloride and depleted brine are fed to saturator 1 and the resulting brine is sent to cell 2 Where is is electrolyzed. Chlorine is produced at an inert anode, potassium is amalgamated by a mercury cathode, and the amalgam ows into decomposer 3 where the potassium reacts with a recycle potassium phosphate solution, producing hydrogen and a mixture of phosphate solution and solids or a Iphosphate solution enriched with KZO and depleted in H2O, The solution or solution with solids is fed into a mix tank 4 where phosphoric acid and water are added in suitable proportions. Solutions and solids are removed as mixed products or are separated by lilter 5 if a solid product is desired. A solution product can also be extracted, or if no solids are formed in decomposer 3 or mix tank 4, only a solution product is withdrawn. Mercury, stripped of potassium, is pumped from decomposer 3 to cell 2; depleted brine is pumped from cell 2 to saturator 1.

Referring now specifically to FIG. 2, there is shown a diagram of equipment We used on tests of a scale smaller than that of a commercial plant and of a size generally referred to as exploratory bench scale. As may be seen, the solid potassium chloride and depleted brine solution were fed to a saturator, operated at room temperature, and the resulting brine was pumped into the electrolytic cell where a current was passed between a graphite anode and a stagnant pool of mercury, manufacturing chlorine at the anode and potassium amalgam at the cathode.

Depleted brine -owed back to the saturator and the amalgam was drain immediately into the decomposer which contained a neutral potassium phosphate solution of nominal 0-28-28 fertilizer grade (0 percent N, 28 percent P205, 28 percent K2O). The reaction produced hydrogen which was collected, measured, and discharged. Water and phosphoric acid were added in the mix tank, and a nominal 0-28-28 liquid product was removed therefrom. Mercury that collected in the bottom of the decomposer was pumped back into the cell so that the cycle could be repeated.

Referring now more specifically to FIG. 3, there is shown a diagram of equipment We used on tests of a scale smaller than that of a commercial plant and of a size generally referred to as bench scale. As may be seen, the solid potassium chloride and depleted brine were fed lto a saturator operated at ambient temperature. The acidity of the brine was monitored at the inlet and outlet of the saurator and appropriate adjustments were made by the addition of KOH or HC1 solutions. The brine was pumped into the cell and a current was passed between a platinum-gauze anode and a owing mercury cathode. Clorine was manufactured at the anode, potassium amalgam at the cathode. The amalgma drained constantly into the decomposer where it was stripped of potassium and was pumped continuously as essentially pure mercury back to the cell. Brine was not allowed to enter the decomposer. Potassium phosphate solution was pumped from the decomposed, mixed in separate streams with appropriate portions of water and with phosphoric acid, which were added intermittently and returned by separate lines to the decomposer. This arrangement minimized degradation of the longer chain-length phosphate species. Nitrogen was mixed with the chlorine that was manufactured, in order to lessen the likelihood of creating explosive mixtures of chlorine and hydrogen n the experimental arrangement; in a commercial plant, nitrogen contamination of the chlorine would not be desired. Hydrogen manufactured in the decomposer was measured in a eudiometer, then discharged. Product potassium phosphate solution was withdrawn from -the surge tank.

Referring now specifically to FIG. 4, there is shown a diagram of equipment we used on tests of a scale smaller that that of a commercial plant and of a size generally referred to as bench scale. As may be seen, the solid potassium chloride and depleted brine were fed to a saturator operated at ambient temperature, the acidity of the brine was monitored at the inlet and outlet of the saturator, and appropriate adjustments were made. The brine was pumped into the cell and a current was passed between a platinum gauze and a flowing mercury cathode. Chlorine was manufactured at the anode, amalgam was produced at the surface of the flowing cathode and drained continuously into the decomposer where it was stripped of potassium and was pumped continuously as essentially pure mercury back to the cell. The resulting phosphate solution or solution with solids overflowed the central well of the decomposer and drained into a quality control unit where the density and pH were monitored as the product was withdrawn. The product also branched to a surge tank. Flowmeters monitored the rates of flow of water and phosphoric acid into the decomposer (burettes measured accumulated flow), the acid entering the lowest level and the water at an intermediate level. This arrangement eliminated the need for a mix tank, minimized degradation of phosphate chain-length, improved circulation within the decomposed, and prevented unwanted formation of solid potassium phosphates within the decomposer. The temperature of the phosphate solution in theV decomposer was monitored with a thermometer placed in a thermometer well. Nitrogen was introduced into the cell where it mixed with chlorine, as the latter was manufactured, in order to minimize the likelihood of creating explosive mixtures of chlorine and hydrogen in the experimental arrangement; in a commercial plant, nitrogen .6 contamination of the chlorine would not be desired. Hydrogen manufactured in the decomposer was passed through an entrainment collector and was delivered to alternate eudiometers where it was measured, then discharged. The use of alternate eudiometers avoided buildup of hydrogen pressure during discharge of the hydrogen.

Referring now specifically to FIG. 5, there is shown a diagram of the decomposer that we substituted for the 4decomposer shown in FIG. 4, supra. This one employed a magnetic spin-bar which served as an agitator and circulator. Introduction of water and phosphoric acid directly into the decomposer eliminated the need for a mix tank. The absence of glass beads or other packing material permitted the formation of phosphate solids, allowing the production of liquids or slurries without obstructing flow of product to the quality control unit. The temperature of the phosphate solution in the ydecomposer was monitored by a thermocouple placed in a thermocouple well; the temperature was controlled by circulating heated water through jackets around the decomposer.

In order that those skilled in the art may better understand how the present invention can be practiced and more fully and definitely understood, the following examples of processes that we have used in the preparation for the production of potassium phosphate compounds prepared according to our invention are given by way of illustration and not by way of limitation.

EXAMPLE I f The decomposer (FIG. 2) was a Z50-ml. borosilicateglass bottle, 2.5 inches I D. by 5.5. inches long fitted with a ground joint for attachment to the electrolytic cell. A batch of potassium amalgam (about g.; 0.003 g. K/g. Hg), manufactured by the passage of 6.0 amperes of electrically for 126 seconds during the electrolysis of saturated potassium chloride solution pumped at 900 mL/hr. by the action of a peristaltic pump upon Tygon tubing, was drained immediately into 500 g. of nominal 0-28-28 potassium phosphate solution. This solution, prepared from potassium hydroxide pellets, water, and percent phosphoric acid, had an actual composition of 28.1 percent P205 and 27.3 percent K2O, Was stirred with a magnetic stirrer, and could be heated if desired. The reaction was essentially complete at 25 C. in 17 minutes. Hydrogen gas was manufactured and delivered to a 1GO-ml. eudometer where a volume of 87 ml., corrected to standard temperature and pressure, was collected. This corresponded to the reaction of `0.30 g. of potassium produced by the cells operating at 99 percent current efficiency. The collected hydrogen was discharged, the mercury, stripped of potassium, was pumped back into the electrolytic cell, and a similar cycle was repeated 9 times. After the addition of 4.9 g. of 105 percent 'I-I3P04 and 5.4 g. of H2O to the phosphate solution, a product weighing 13.4 g. had been manufactured. It had the identical composition of the initial phosphate solution; the density was 1.63 g./ml. at 25 C. and the pH was 7.02. The phosphate species distribution was: ortho, 55 percent; pyro, 39 percent; tripoly, 5 percent; and other, 0.4 percent. This distribution was the same as that of the original 0 28-28 mix.

EXAMPLE 1I The decomposer (FIG. 3) was a Z50-ml. borosilicate- Y glass bottle into which a thermometer well was built so that the temperature of the decomposition-reaction could be followed. The electrolytic cell incorporated a well into which freshly formed potassium amalgam flowed continuously into the decomposer. Mercury or amalgam that collected in the bottom of the decomposer was pumped back to the cell. In one set of tests, the decomposer was charged with 400 g. of distilled water; the brine, mercury, and water were pumped at 900 ml./hr. by the action of peristaltic pumps on Tygon tubing, and a current was passed through the brine. Hydrogen that was manufactured was delivered to a 10U-ml. eudiometer, but because the decomposer did not contain graphite to accelerate the rate of decomposition, the reaction between the amalgam and the water was so slow that after an hour only 5 ml. of hydrogen had been manufactured although enough umn 2O inches long with a central reactor column, filled with glass beads, into which water, phosphoric acid, and potassium amalgam were continuously introduced after an initial charge of pure water. The brine and the mercury were separately pumped at 900 ml./hr. and current potassium had been amalgamated to manufacture more 5 was passed through the brine. Amalgam continuously than a liter of hydrogen. The calculated pH was 11. When drained into the decomposer and potassium phosphate 2 g. of 105 percent H3PO4 was added, a vigorous reaction solution was produced, overliowed the central reactor took place, as evidenced by the rapid evolution of hy- Column, and drained into the quality control unit where drogen. In 12 minutes, 269 ml. of hydrogen, corrected to 10 the pH and density were monitored. The pH of the liquid standard temperature and pressure, was collected. Thereis a measure of the ratio K2O: P205; it was raised either after, H3PO4 was added intermittently at such a rate as by increasing the amount of current in the electrolytic to approximate a KZC/P20 weight ratio of 1.1 within cell or by decreasing the rate at which phosphoric acid the decomposer. As the electrolysis continued, appropriate was added. Depending upon the product desired, the opadditions of KOH or HCl to the brine were made to main- 15 eratiug range of pH is 0-14, but `the preferred range is tain the desired acidity. Typical operational temperatures 5-10. Operation at pH 5 or below causes hydrolysis of attained in the system when the current was 11 amperes POYPhOSPhateS t0 OfthOPhOSPha'S; Operation at PH 10 were: decomposer, 50 C.; brine saturator, 40 C.; and or above slows the rate of decomposition of potassium cell-brine, 70 C. The data which were acquired from amalgam, and requires a short-circuited decomposer as is th ensuing tests are shown in Tables I and II below; these Sed 111 amalgam ChtOf'alkati Plants- Limited informadata demonstrate that the phosphate solution is effective tOIl 011 SOtUblteS 0f mXtUleS 0f Potassium phosphates in decomposing amalgam and that, as the concentration SJggStS that the mXmUm SOlllblitieS Will be in the pH of the phosphate increased, the distribution of species was ranges 6-10, COffCSPOndUg t0 ratios 0f 1420213205 0f maintained, 0.9-1.3, respectively. The density is a measure of the total TABLE I plant food and was controlled by the rate of addition [Electrolysis Ofpum KClSolutionin amercury can] of water to the decomposer. Projections show that densities of pure solution range as high as 1.9 g./ml. at 25 Charge, Hydrogen Theorl' Current C. for 70 percent total plant food. Such solutions would Current, couconeeted, hydrogen efficiency, have a high order of stability even though some may be amperes lambs m1. (STP) ml. (STP) percent supersaturated. The data which were accumulated from E azg ggg S21? the tests are shown in Tables III and 1V below. During 6 36:;,20 3:452 j255 81 the first three runs, 104.9 percent H3PO4 (76 percent 10 121.800 14,296 14,147 101 P205) furnace acid was used. For the other runs the mi? lggjgg 123g g) acid was 108.1 percent H3PO4 (78.3 percent P205) made from a mixture of furnace acid and clarified wet-process Total 408'080 44'910 47399 95 acid. The phosphate product was a clear colorless liquid,

TABLE II [Potassium phosphate production] Phosphate species distribution,

K10 P205 percent; Density,

Cumulative P205, KzO, weight g./rnl.

ampere-hours percent percent ratio Ortho Pyro Trpoly Other pH at 25 C EXAMPLE III The decomposer (FIG. 4) was a borosilicate-glass col- Only the primer and the liquids from runs 1 through 3 developed crystals upon standing at room temperature.

TABLE HI [Electrolysis of pure KCl solution in a mercury cell] Hydrogen, liters Total Average Current Current, Time, charge, current, Theoretiemciency Run No A hr. Ah A RTP STP cal, STP percent TABLEIV [Potassium phosphate production] Phosphate species distribution, Composition, Weight percent of total P Density, Weight, P.p.m. ratio, p at g. ml. Run No. g P205 KzO Cli Hg K2O:P2O5 Ortho Pyro Tripoly Other 25 C. at 25 C.

Primer solution.- 1, 03a 27.6 28. 2 0.01 0.01 1. 022 60.1 37.2 2. 1 0. 5 7. 22 1. 637 1 695 27. 5 27. 9 0. 04 0. 01 1. 015 60. 0 36. 3 3. 0 0. 7 7. 14 i. 625 2 541 23. 5 23. 5 0. 01 0. 12 i. 000 58. 9 37. 3 2. 9 1. 0 6. 60 1. 504 3.-.. 513 21. 8 24.8 0. 66 0.01 1, 138 64. 9 30. 8 3. 4 0. 9 7. 44 1. 505 297 23. 5 26. 3 0. 59 0. 01 1. 119 57. 9 34. 2 6. 6 1.3 7. 53 1. 553 309 25. 1 26. 9 0. 60 0. 13 1, 072 45.3 39. 1 12. 4 3. 2 7. 33 1. 57s

289 25. 5 27. 5 0. 53 0. 15 1. 07s 40. 7 40. 2 14. 9 4. 2 7. 45 i. 594 651 24. 6 27. 2 0. 90 o. 63 1, 106 37. 2 40. 9 16. 9 5. 0 7. 45 1. 576 659 25.4 27. 2 0.89 0. 20 1. 071 30. 1 43. 4 19. 4 7. 0 7. 24 i. 588 56s 26. 7 2s. 2 0. 59 0. 18 1. 056 28. 4 43. 9 20. 4 7. 3 7. 39 1. 616 481 28. s 28. 7 0. 38 0. 19 0. 997 29. 4 45. 3 19. 4 5. 9 7. 14 i. 660 960 26. 2 29. 0 0. 32 0. 27 1. 107 26. 8 45. 1 20. 6 7. 4 7. 93 1. 634

EXAMPLE IV The borosilicate-glass decomposer (FIG. 5) employing a magnetic agitator-circulator was substituted for the one TABLE VI shown in FIG. 4, supra. Amalgam and phosphoric acid were fed at the top and Iwater was fed near the bottom. Production of Potassium Polyphosphate Solution The mercury and the brine were separately pumped at 1800 ml./hr. Hot water introduced into jackets fixed Welght, g. 535 the decomposition temperature at 70 C. The feed acids Composition, percent employed were wet-process acids prepared from calcined P205 30.9 North Carolina phosphate rock; these acids had a 2 per- K2O 31.1 cent metallic contamination and were green in color. The Weight ratio results are shown in table V below. The products were 1920511205 1.006 slurries which, upon settling, consisted of a pale green pH 767 solution and solids consisting primarily of Density g /m1 at 25 C 1740 (A1,Fe)PO4xH20 EXAMPLE V1 and Mg2KH(PO.2)2-15H20. Wet solids constituted about 35 The System Used in Example IV, supra, was employed.

10 percent of the production during the first 8 runs when 75 percent H3P04 (54 percent P205) was used, but only about 1 percent of the production during the last 2 runs when 99 percent H3PO4 (72 percent P205) was used. The phosphate in the former acid was 0 percent polyphosphate whereas that in the latter was 48 percent polyphosphate. Unwanted solids could be avoided by mixing the impure wet-process polyphosphoric acid with pure furnace polyphosphoric acid.

The feed acid was reagent grade 85 percent orthophosphoric acid (62 percent P205). The results are shown in Table VII below. The concentrated potassium orthophosphate solution that was produced contained as much as 48 percent plant food (P205 percent-i-K20 percent) and was a stable liquid when stored at room temperature. It contained no more than a trace of polyphosphate.

TABLE V [Potassium phosphate production] TABLE VH Solution Production of iitassiuirurorthophosphate solutions Wighg of C siti OmpOSl 10U, pro ue s, g. ompo on P t h R S 1 W t percent Weitght ercen Wrtltgio Dag/11g: lm Oll' e T8101 P0 O KOzP No. non soiids P205 Kzo Kiozot pH 2 5 K2 2 O5 pH at 25 C 17.7 18.1 1.023 7.10 1. 380 10.3 10.5 1.02 6.26 23.6 24.7 1.047 7.49 1.555 g 2.5 .22g 24.2 1.076 7. 4s 1. 531 21:3 25:9 1:22 7:48 .7 24.2 1. 066 7.47 1.530 21.5 25.1 1.17 7.25

17. 8 21. 0 1. 18 7. 53 22. 8 26. 0 1. 14 7. 47 t? it? tt 2113 263 1,'23 7.`67 f EXAMPLE VII j '522g jg The system employed for Example -I V, supra, was used. 26.8 29.3 1-09 7-88 1659 Very pure brine is normally required in a mercury cell if a high current efiiciency for the production of amalgam is to be attained. The usual current eiciency when NaCl EXAMPLE V 65 is used is -97 percent and when KCl is used is 92-94 percent (Electrolysis of Brines in Mercury Cells, by R. B. MacMullin, page 182 in Chlorine-Its Manufacture, Properties and Uses. I. S. Sconce, editor, Reinhold Publishing Corporation, New York, 1962). Three grades of potassium chloride were dissolved and electrolyzed in a mercury cell. Brine A was made from a very pure reagent grade KCl; brine B was made from coarse I-Iigran KCl; and brine C was made from granular Red KCl. Analyses of the solid potassium chlorides are shown in Table VIII.

Some of the brine was treated with orthophosphoric acid (62 percent P205), others, with polyphosphoric acid (80 percent P205). The results are shown in Table IX, infra. Orthophosphoric acid in the concentration range 0.3 to 1.8 percent P and polyphosphoric acid in the concentration range 0.01 to 2.0 percent P were found to have a beneficial effect upon the current efiiciency.

TABLE VIII [Analysis of solid potassium chlorides] Composition, Wt. percent U.S. Borax and Chemical Company J. T: Baker 62-63% K10 59-61% KzO Chemical Co. coarse granular reagent-grade Higran" Red Component:

KC 99. 6 98. 8 93. 97 KzO- G2. 9 62. 3 59-61 0. 94 1. 8-2. 5 007 0. 03 0. 08 0. 03 0. O9 0. 02 (l. 08 0. 17

0. 03 0. 15 0. 01 0. 05 ND ND 0. l 0. 0l ND 0. 05-0. 10 0. 05 0. 0. 0l 0. 01 ND ND ND 0. 05 0. 01 0. (ll 0. 0l 0. 0l ND Nl) 0. 46 0. 7-1. 4 H2O 0. 28 0. 20 Other compounds 0.03 0. 82 pIl of saturated solution at 25 C 5. 5 8. 6 l0, 0

NOTE.-TR=trace; ND =not detected by spectrographic analysis.

TABLE IX [Electrolysis of KCl solutions in a mercury cell] Brine Current l A=Reagentgrade KCl; B="Higran Kol; C=Red" Kol.

2 0=No treatment; l=orthophosphoric acid; 2=polyphosphoric acid.

After combing and panning the data, results, and operations of our new, novel, improved, and unique method for producing chlorine gas, hydrogen gas, and potassium phosphate solution, mixture of solution and solids, mixture of solution and mixture of solids, particulate phosphate, or mixture of particulate phosphates, all phosphates essentially free of chlorine, we now present in Table X below the acceptable and preferred ranges of operating the variables involved when water is saturated with solid potassium chloride, the resulting solution is electrolyzed in a mercury cell to produce chlorine gas and potassium amalgam, and said amalgam is subsequently decomposed in a potassium phosphate solution to which is also added phosphoric acid and water, and from which potassium phosphate and hydrogen gas are produced and mercury is reclaimed.

TABLE X Production of chloride-free potassium phosphates acceptable and preferred ranges oi variables Reaction variable Limits Preferred Saturator temperature, C 25435 50430 Feed brine pH 2. 5-5. 5 3. 4-4. 5 Feed brine temperature, C 40-70 50-60 Concentration of potassium chloride in feed brine, g./l 330-360 340-350 Concentration of orthophosplloric acid in brine,

percent P 0.1-2.5 0.5-1 O Concentration of polyphosphoric acid in brine,

percent P 0. 01-2. 0 050. 3 Cathode current density, A/cm.' 0. 1-3. 5 0. 20. 7 Anolyte temperature, C 60-90 70-80 Anolyte pH 2. 0-4. 0 2.5-3.5 Concentration of potassium chloride in spent brine, g./l 250-300 270-280 Concentration of potassium in amalgam, Weight percent 0. 1-0. 5 0, 20-0. 35 Concentration of P205 in acid, weight percent 3.0385 50-80 Phosphoric acid temperature, C IO-boiling 50-60 Process water temperature, C.. ltl-boillng 25-60 Decoinposer temperature, C. 35400 Acid, 30-70% P2O5. 65-95 Acid, 70-87% PQO; 50-70 Decomposition pH 014 5-10 While we have shown and described particular embodiments of our invention, modifications and variations thereof will occur to those skilled in the art. We wish it to be understood therefore that the appended claims are intended to cover such modifications and variations which are within the true scope and spirit of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A process for the production of chloride free potassium phosphate fertilizer which comprises the steps of:

(l) simultaneously introducing into a saturator a stream of solid potassium chloride and a stream of depleted brine solution, said brine solution recycled from a later-mentioned electrolytic cell;

(2) removing from said saturator the resulting brine saturated with potassium chloride and introducing same into an electrolytic cell together with a stream of mercury, said mercury flowing in juxtaposition with the cathode portion thereof;

(3) applying direct current across said electrolytic cell of voltage and amperage sutiicient to electrolyze said saturated potassium chloride brine therein; removing gaseous chlorine therefrom formed at the anode thereof, returning the resulting depleted brine to said saturator, and removing the resulting mercury potassium amalgam formed at and in the mercury cathode therein;

(4) introducing said amalgam removed from said electrolytic cell into a decomposer together with potassium phosphate solution recycled from a later-mentioned mixer; removing from said decomposer a stream of mercury stripped of potassium and recycling said mercury back to said electrolytic cell; offgassing from said decomposer the resulting hydrogen gas; and removing from said decomposer a stream of resulting potassium enriched water-depleted phosphate solution;

(5) introducing said stream of potassium enriched water-depleted phosphate solution into a mixer, together with a stream of phosphoric acid and a stream of makeup water; and

(6) removing from said mixer the resulting chloridefree potassium phosphate solution, a portion of which is recycled to said decomposer and the remainder of which is collected as product.

2. The process of claim 1 wherein at least a portion of the resulting potassium enriched material withdrawn from said mixer is introduced to filter means wherefrom is removed as solids a chloride-free potassium phosphate fertilizer first product and wherefrom is removed as liquid a chloride-free potassium phosphate solution, a portion of which solution is recycled to said decomposer and the remainder of which solution is collected as a second product.

3. The process of claim 1 wherein said saturator is operated in the temperature range from about 25 C. to about 65 C.

4. The process of claim 1 wherein said saturator is operated in the range from about 50 C. to about 60 C. and ,wherein said electrolytic cell is operated at a current density of about 0.1 A/cm.2 to about 3.5 A/cm.2.

5. The process of claim 3 wherein said saturator is operated at about 55 C, said decomposer is operated at about 35 C. to about 100 C., and said electrolytic cell is operated at a current density of about 0.2 A/cm.3

. to about 0.7 A/cm.2.

References Cited UNITED STATES PATENTS 3,681,214 8/ 1972 Iones cu et al. 204-99 14 l FOREIGN PATENTS 1,247,198 9/ 1971 Great Britain.

772,226 4/ 1957 Great Britain' 204--99 391,673- 9/ 1965 Switzerland 240-99 1,266,289 4/ 1968 Germany 204-220 4,412,259 3/ 1969 Japan 204-99 SAMIH N. ZAHARNA, Primary Examiner R. BARNES, Assistant Examiner U.S. Cl. X.R. 204-99; 423-180, 309, 313

PO-wo UNITED .STATES PATENT OFFICE 7 A CERTIFICATE OF CORRECTION Patent No. 3,832,156* Dated August 27, 1974 Inventor). Travis P. Hignett and Anthony J.' Smith It is certified that error appears in the above-identified patent and that said Lettersv Patent are hereby corrected as shown below:

Column 1, line 58, change formula to: [KmH(n m+2)PnO(n+-1)] H20 line 61, changeV formula to:

Column 2, line 48, before "concentration" insert al Column 3, line 37, change "particular" to particulate line 62, change "precent" to percent C `Column 4, line 5, change "fround" to found Co1umn5, line 2, change "drain" to drained f; line 18', change A "saurator" to saturator line 22, change""C1orine" to `Ch1orine line 23, change "amalgma" tov amalgam ;l line 28, change "decomposed" to decomposer line 67, change "decomposed" to decomposer Column 6, line 35, change "electrically" to electricity --A line 20, "th" should be --4 the Column 10, Table IV, under Vheading "Weight ratio, K2O:P205" fourth figure should be 1.138 instead of 1,138; sixth figure should be 1.072 instead of 1,072.; eighth figure should be 1.106 instead of 1,106. Column l1, Table' VIII, under heading "S9-61% R20 granular Red" change the third item from 1.8-2.5 to 1.8-3.5

Signed and sealed this 19th day of Novem-ber." 1974.

(SEAL) Attest:

`MeCOY M. GIBSON JR. I C. MARSHALL DANNl v Attestng Officer Commissioner of Patents 

